Lead-activated BaSi.sub.2 O.sub.5 (BaSi.sub.2 O.sub.5 :Pb)is used as the long-wavelength UV (347 nm) phosphor in fluorescent suntanning lamps. Its preparation and use have been described in U.S. Pat. Nos. 2,597,631, 3,043,781, and 5,234,625. A well-known problem with the use of this material is that its radiance maintenance in a fluorescent lamp is poor relative to other fluorescent lamp phosphors. That is, suntanning lamps employing the BaSi.sub.2 O.sub.5 :Pb phosphor exhibit a relatively large decrease in radiant output over time. The reasons for the poor radiance maintenance of this material are still not well understood. It has been demonstrated generally that the maintenance of certain phosphors may be improved by modifying the phosphor surface or applying a protective coating. In particular, U.S. Pat. No. 5,695,685 describes the application of an ion-exchange treatment to a variety of lamp phosphors. However, the improvement in lamp maintenance is only about 2% and no examples are provided of the successful application of the method to BaSi.sub.2 O.sub.5 :Pb phosphor. In other prior art, U.S. Pat. No. 5,223,341 describes a conformal Al.sub.2 O.sub.3 coating formed by an MOCVD technique which increases the radiance maintenance of a BaSi.sub.2 O.sub.5 :Pb phosphor at 100 hours of lamp operation from 75-85% up to 90-95%. However, the method of application of the coating by MOCVD techniques generally is more expensive than solution based methods. Thus, it would be an advantage to have a solution based method which would produce a significant improvement in lamp maintenance.